In a usual facsimile transmission, a sending terminal reads an original image as multivalued image data by an incorporated scanner, converts the data into binary image data and sends the converted binary data.
The receiving terminal outputs the received binary image data to print the data on recording paper.
The conventional image processing device converts multi-valued image data into binary image data in the following way:
First, the original image is divided to character image areas and halftone image areas.
Means of discriminating between the character image areas and the halftone image areas are usually called as area deciding means.
Area decision is made firstly by examining whether density of an objective pixel to be examined is within a range between a specified black threshold and a specified white threshold and secondly by examining whether a difference of the density of the objective pixel from an average optical density of four pixels existing in upper, lower, left and right of the objective pixel is equal or smaller than a specified threshold. The objective pixel is decided as a halftone area if it satisfies both conditions as the result of the examinations. The objective pixel is decided as a character area if it does not satisfy both conditions at a time as the result of the examinations.
The halftone pixel thus decided is binarized by dithering with error diffusion method. A pixel judged as a character area pixel is binarized by using a specified threshold value, omitting dithering process that may impair character information.
A halftone image represented by halftone dots of a larger size than a certain value of roughness cannot be processed without causing moire fringes by a conventional image processing device. Moire fringes may be produced because the conventional image processing device may periodically mistake an image represented by halftone dots for a character image.
The conventional image processing device decides an area of each of the pixels read by scanning the original image by referring to four pixels existing just upper, lower, left and right from each objective pixel. In this case, each objective pixel is judged to be of a character area if a difference of its density from an average density of the referred pixels exceeds a certain threshold value.
In this halftone dot image, pixels at positions of a certain cycle are misjudged to be of a character area because the differential values of its density from an average density of the referred pixels cyclically exceed the threshold value. These misjudgments cause moire fringes in the image when reproduced.